Car comparison calculator
The sticker price is the easy bit. What a car actually costs you is the sticker price minus what you sell it for, plus everything you spend in between, and that second part is usually the larger number.
This compares two vehicles on the full picture: depreciation, fuel or electricity, road user charges, servicing, insurance and loan interest. Petrol, hybrid, diesel, EV or plug-in hybrid, in any combination.
Enter your details in the purple cells. It's your sheet to keep and change.
Depreciation is the cost nobody budgets for
Ask most people what their car costs them and they'll tell you what they spend on petrol.
But a $40,000 car sold for $16,000 ten years later has cost you $24,000 in depreciation. $2,400 a year, or roughly $46 a week, before you've bought a single tank of fuel. On most vehicles it's the single biggest line, and it's invisible because it never leaves your bank account.
That's why the calculator works from purchase price and expected sale price rather than just running costs. Guessing the resale value is imprecise, but leaving it out is worse.
Comparing an EV or hybrid against a petrol car
This is where the arithmetic gets genuinely awkward, and it's why most comparisons you'll find online don't work here. They're American, so they're in miles, gallons and MPG, and they have no concept of road user charges.
Petrol and non-plug-in hybrids pay no RUC. The road charge is already inside the pump price as fuel excise. If you drive a Prius or an Aqua, you've nothing to buy.
Diesel and full EVs pay $76 per 1,000km. On 15,000km a year that's $1,140 you weren't paying before, plus an admin fee every time you buy a licence.
Plug-in hybrids pay a reduced $38 per 1,000km, because they still buy petrol and pay excise on it. Charging them the full rate would be taxing the same kilometre twice.
The calculator applies the right treatment automatically once you pick the fuel type. For a plug-in you also set what share of your driving is on electricity, since that's what decides the split between charging cost and petrol cost.
Rates as at August 2026. Check NZTA if it's been a while. The Rates tab in the spreadsheet is editable so you can update it yourself.
An efficient car pays less road tax
Worth knowing, because it's the argument behind where this is all heading.
Fuel excise is a fixed amount per litre. So a thirsty car pays a lot of road tax and an efficient one pays little, for exactly the same distance and the same wear on the road. A hybrid doing 4L/100km contributes noticeably less per kilometre than a diesel or an EV paying the flat $76.
The government has signalled it intends to move every vehicle onto RUC and scrap petrol excise altogether. If that happens, the maths in this calculator changes for petrol cars too, which is why the rates sit on their own tab where you can edit them.
What the calculator covers
Purchase price and expected resale.
Fuel or electricity, priced properly for whichever you're using.
Road user charges including the admin fee.
Registration, warrant, servicing and insurance.
One-off costs like tyres or a cambelt, spread across the years.
And loan interest, if you're borrowing. The interest only, since the car's price is already counted in the depreciation.
Out the other side: cost per year, cost per week, and total cost over the whole time you own it.
Plus the number people actually want when they're weighing up a hybrid or an EV: how many years the cheaper running costs take to pay back the extra you spent buying it.
That last one tends to settle the argument. A vehicle that saves you $1,800 a year but costs $15,000 more up front takes over eight years to break even, which may be fine if you're keeping it fifteen years, and isn't if you're keeping it four.
What it can't tell you
Fuel and electricity prices over the next decade. Nobody knows those, and they move the answer considerably. Worth running it twice. Once with power cheap and petrol dear, once the other way, and seeing whether the answer flips.
Resale value, which is a guess for any car and more of a guess for an EV, where battery condition and how fast the technology moves both matter.
Reliability. A cheaper car that spends a fortnight a year off the road has a cost the spreadsheet can't see.
Whether you'll actually like driving it, which matters more than most money people admit.
Thinking about a change?
A car is usually the second-biggest purchase people make, and it's the one most often decided on the forecourt rather than at the kitchen table.
If you'd like to talk it through, including whether to buy outright, finance it, or keep what you've got a bit longer — get in touch for a free 30 minute chat.
Independent, commission-free, and I don't sell cars or car loans.